The purpose of the proposed study is to determine the role of the liver, kidney, gut, mammary gland, placenta, and fetus in retinol metabolism of sheep given various intakes of vitamin A. The study will progress from evaluation of liver, and kidney responses in catheterized, non-gravid sheep administered 3H-retinol to more advanced studies of pregnant ewes involving administration of 3H-retinol to catheterized ewes and fetuses. Hepatic and renal arterial and venous catheterization in ewes and fetal lambs, followed by 3H-retinol intravenous injection, will provide detailed and quantitative data concerning vitamin A uptake, storage and mobilization. Analyses of free 3H-retinol, 3H-retinol-retinol binding protein, and retinyl esters in blood and tissues will help to determine factors controlling vitamin A utilization. Deconvolution and compartmental analysis will be employed to construct a mathematical model of the vitamin A system during pregnancy, fetal and neonatal life.